It's not actually, look up some photos of the sun setting over the ocean. Here's an example:

https://stockcake.com/i/sunset-over-ocean_1317824_81961

That’s only if the sun is above the horizon entirely.

No, it's not.

https://stockcake.com/i/serene-ocean-sunset_1152191_440307

Yes, it is. In that photo the sun is clearly above the horizon, the bottom half is just obscured by clouds.